Moral Dilemma

I'm reading this absolutely amazing book right now (more on the book once I finish it). But since it's from the library, I can't underline all its wonderful passages. And I so want to underline all its wonderful passages. I'm trying to decide if I should just give in and get out my pen, and pay whatever fine the library imposes on me. I'm only a quarter through the book, and hope I have the moral fortitude to not start marking the text. If I happily pay the library's fine, is it still considered immoral? Is it wrong to treat one's public library system like a trial offer?